1. Field of the Invention
The present invention relates to DC-DC converters in which an inductor and a switching control IC are provided on an insulator substrate.
2. Description of the Related Art
In cellular phones, multifunctional portable terminals, or the like, a plurality of voltages are required to support multiple frequency bands, and non-isolated DC-DC converters are used. A DC-DC converter includes a coil, capacitors, and a switching IC. An example of an existing DC-DC converter has a configuration in which a coil pattern is formed in an insulator substrate and capacitors and a switching IC are mounted on the surface of the insulator substrate.
In such a configuration, since a DC-DC converter includes a switching IC mounted on the surface of an insulator substrate, the dimension of the DC-DC converter in the thickness direction of the substrate is increased. Accordingly, a smaller (thinner) DC-DC converter is desired in cellular phones, which have become increasingly thinner. Hence, a DC-DC converter has been proposed which is made thinner by using a configuration in which a switching IC is formed in an insulator substrate and a coil and capacitors are formed on the surface of the substrate.
In addition, since high-frequency noise (switching noise) is generated during the switching operation performed by a switching IC in a DC-DC converter, it is necessary to take measures against such noise. In Japanese Unexamined Patent Application Publication No. 6-152079, a printed wire board is disclosed that is used for mounting electronic components and that can realize a stable operation even in an environment where electromagnetic noise is generated from the electronic components. Although most of the electromagnetic noise can be removed using a filter formed of, for example, a coil or a capacitor, there is a problem in that no mounting space is available. Hence, in Japanese Unexamined Patent Application Publication No. 6-152079, a magnetic material is pasted around holes that are used to attach the leads of electronic components. This realizes an effect similar to that realized when coils are inserted in series with the electronic components, whereby the electromagnetic noise is removed.
However, in a DC-DC converter, a ground electrode pattern is usually formed on the bottom surface of an insulator substrate as a common electrode for a coil, a switching IC and the like. Since components are connected to a common ground electrode pattern, switching noise generated by the coil and switching IC leaks from a ground line. This problem is not solved by the invention disclosed in Japanese Unexamined Patent Application Publication No. 6-152079.